Electrostatic discharge (ESD) is one of the most destructive phenomena in modern industry, especially the electronics industry. Electrostatic discharge is defined in the U.S. Military Handbook DOD-HKPK-263 as a transfer of electrostatic charges between bodies at different potentials by direct contact or induced by an electrostatic field. Electrostatic charge can build up on non-conductive materials as the result of the capture or release of electrons. A non-conductive material can capture or release an electron by rubbing or heating it, or it can become charged through contact with another previously charged object. For example, if a non-conductive material captures an electron from one of the sources listed above, it will become negatively charged and will hold that charge. If a positively charged body comes into contact with the negatively charged non-conductive material or comes within the static field, an arc may be discharged from the non-conductive material to the positively charged material in order to dissipate the buildup of the electrical charge. This transfer of electrostatic charge causes destruction and damage to electronic components estimated at millions of dollars a year.
One of the primary generators of ESD is the movement of personnel or equipment across the floor in a manufacturing area, assembly area or shipping area. Static damage to integrated circuit components by operating personnel is becoming one of the most significant problems plaguing the electronics industry. Table 1 below represents typical electrostatic voltages generated for various types of movement:
TABLE 1 ______________________________________ MEANS OF ELECTROSTATIC VOLTAGES STATIC 10-20% Relative 65-90% Relative GENERATION Humidity Humidity ______________________________________ Walking across carpet 35,000 1,500 Walking over vinyl floor 12,000 250 Worker at bench 6,000 100 Mobile storage carts On vinyl floors 5,000 ______________________________________
Frequently, little attention is given to the ESD control of a floor or workbench that is near or adjacent to an ESD controlled area. It is not uncommon to assume that operating personnel will be sufficiently grounded with a wrist strap and therefore, the floor or workbench need not be grounded or protected from ESD. However, various non-operating personnel and equipment that may not be sufficiently grounded move in and out of work areas and interact with operating personnel, thus creating ESD and the destruction of sensitive electronic components.
As device technology advances to achieve higher speeds and greater functional density, device geometries are decreasing, line widths and spaces are getting smaller and oxide layers are getting thinner. As a result, lower voltage and current will damage sensitive electronic components. Although electronic components are most vulnerable at the chip or integrated circuit stage, when numerous integrated circuits are assembled on a board or even a final product, damage can occur. Table 2 represents the susceptibility ranges of various electronic devices:
TABLE 2 ______________________________________ RANGE OF ESD SUSCEPTIBILITY DEVICE TYPE (VOLTS) ______________________________________ Bipolar transistors 380-7,000 CMOS 250-3,000 EPROM 100 Film resistors 300-3,000 (thick, thin) GaAsFet 100-300 MOSFET 100-300 OP-AMP 190-2,500 SAW 150-500 Schottky diodes 300-2,500 Schottky TTL 1,000-2,500 SCR 680-1,000 VMOS 30-1,800 ______________________________________
In light of the potential damage that ESD can cause to electronic components, various ESD protective coatings have been developed to be applied to non-conducting substrates to make them conductive or static dissipative. A substrate that is conductive has an electrical resistivity in the range of about 10.sup.0 to about 10.sup.5 ohms/square, whereas a substrate that is static dissipative has an electrical resistivity in the range of about 10.sup.5 to about 10.sup.12 ohms/square. Within the conductive or static dissipative range, static potentials may be dissipated without harming the electronic component. However, a substrate with an electrical resistivity in the conductive range may pose harm to operating personnel. Further, a substrate with an electrical resistivity of greater than 10.sup.12 ohms/square is considered to be insulative and highly destructive with respect to electronic components. Therefore, it is desirable to have an ESD protective coating that is within the static dissipative range.
The most common type of ESD protective coating in the electronics industry is based on ionic conduction, like that disclosed in Great Britain Patent No. 2,148,915A to Berbeco. Ionic conduction involves a humidity dependent coating in which small amounts of moisture are needed to allow for the migration of ions and hence the overall flow of electrons. Ionic conduction based coatings are conductive (10.sup.0 to 10.sup.5 ohms/square) or static dissipative (10.sup.5 to 10.sup.12).
However, there are shortcomings to ionic conduction based ESD protective coatings. For example, these coatings are humidity dependent. Ionic protective coatings are made up of molecules that have an atom with a positive or negative charge on one end of the molecule, such as a sodium or chlorine atom, which attracts water molecules. When the humidity is high, the abundance of water molecules will be attracted to and will build up on the protective coating. When water molecules are allowed to build up over the surface of the ionic coating, the innate conductivity of the water film dissipates the buildup of any accumulated electrical charge, thereby preventing an unwanted electrical static discharge. However, if the humidity drops below 20%, ionic conduction is drastically minimized such that operating technicians, just walking across the floor to the workbench, will build up a static electric charge sufficient to damage an electronic component (See Tables 1 and 2). Another disadvantage to humidity dependent ionic conduction is that when the humidity decreases to about 20%, the air itself, being dry, becomes part of the electrostatic buildup mechanism every time there is an air flow (e.g., wind, air conditioning, blower) passing over an insulated substrate.
Yet another disadvantage of ionic conduction protective coatings is that they are easily degraded by contact with water or detergent solutions. The protection provided by an ionic conduction based protective coating is greatly reduced if the coated substrate is washed with water or a detergent solution. Therefore, the substrate in need of protection must be constantly recoated.
As will be discussed below, the present invention has the technical advantage of providing ESD protection without concern for the level of humidity. Because the present invention is water insoluble, it has the technical advantage of allowing coated substrates (e.g., floors, workbenches, etc.) to be washed or cleaned without removing the ESD protective coating. Moreover, the composition can be applied to objects with identification numbers, etc. because it forms a clear film when dried.